1. Field of the Invention
The present invention relates to a screw joint for a drill run for percussion rock drilling.
2. Description of the Related Art
Screw joints of this nature are used to join together drill string elements, such as a drill bit, one or more drill strings and a shank adapter. The screw joints include mutually co-acting male and female screw threads where an external male thread on a component element is screwed to an internal female thread on a co-acting element, so as to join said elements together.
Swedish Patent Specification 515518 teaches a screw joint of the aforesaid kind in which the male and the female threads are conical and the tops and bottoms of the thread profiles have a radius of curvature that is greater than 30% of the thread pitch. Such a screw thread is referred to typically as a rope thread.
Although such a thread functions satisfactorily in the case of drill strings of more slender dimensions, problems occur as the dimensions of the drill strings increase, resulting in shortening of the useful life of the thread. This is mainly due to the tensile stresses acting on the internal thread, causing the screw joint to be tightened excessively, or over-tightened. In turn, this makes it difficult to loosen or disconnect the joint when drilling has been terminated or when fresh drill rods shall be connected up.